If it is difficult to understand the speaker, it will be difficult or impossible to provide a caption

Captioning will not be perfect, but owners can edit or upload their own caption

It looks like captioning is an “opt out” function;. That is, the caption is shown by default; you have to turn it off if you don’t want it. Here’s an example (hover over the arrow in the bottom right):

Twenty hours of video is uploaded per hour — it will take a while for this to be available across all videos, but won’t it be great when it is! In addition to providing additional accessibility, it will also provide additional text for search engines, so video will be found much more easily.